CDMA cellular communication technique shows great potential for its features associated with large capacity, simple frequency planning, good communication quality and small electromagnetic interference. CDMA spread spectrum receivers are classified into coherent receivers and non-coherent receivers. Coherent receivers need the phase information of received signals, and non-coherent receivers do not need the phase information, but need the transmitted signal in quadrature modulating. The present invention pertains to coherent receiving mode in future CDMA cellular communication systems.
Multipath fading which causes serious multipath interference exists in a mobile communication system. In general, it is necessary to receive pilot signals with confirmation information so as to evaluate the amplitude and phase information of multipath signals so that it is possible to achieve multipath diversity and coherent reception. A coherent spread spectrum receiver which performs diversity processing is referred to as RAKE coherent receiver. RAKE coherent receivers can correct phases of a plurality of singlepath signals which carry same information and are independent from one another in fading features, and perform maximal combination to overcome multipath fading and improve received signal-to-interference ratio.
To achieve RAKE reception function, Synchronizing local spread spectrum sequence (PN code) with received signal is necessary. The synchronization is achieved by acquiring and tracking steps. The acquiring step acquires a pilot channel and confirms that initial synchronization (coarse synchronization) of PN code is complete. The tracking step finely synchronizes the PN codes by tracking pilot signals. The combination of these two steps provides PN code and accurate local timing required for a RAKE receiver.
Conventional PN code tracking method is based on so-called “early-late-gate” technique which finely adjusts phase of local PN code by observing and comparing the average energy in lead timing and lag timing of current pilot signals so as to finely synchronize PN codes. In the case where multipath arrival signals exist, effective arrival signals from each of paths are subjected to “early-late-gate” processing to achieve fine synchronization to multipath signals. Unfortunately, random changes in both the amplitude and phase of the arrival signals from each of the paths occur in a large range due to the serious fading in mobile communication systems. The tracking procedure for signals from each of the paths became extremely unstable as the fading of the signals if conventional “early-late-gate” technique is used. Further, the local timing for a receiver is generally established based on the signals from the earliest effective arrival path. Therefore, the establishment of local timing for a receiver lacks stability because the signals from the earliest effective arrival path are extremely unstable.